Some engines may include an exhaust system having one or more aftertreatment devices. As one example, a diesel engine may have an exhaust system that includes a diesel particulate filter (DPF) for removing particulate matter from the exhaust passage prior to exhausting the gases produced by the engine to the surrounding environment. During some operations, a DPF may burn off built-up filtered particulate matter, thereby regenerating the filter. Regeneration may occur passively under conditions where sufficient exhaust heat is generated by the operation conditions. Alternatively, or in addition, exhaust gas temperature can be increased via engine measures and/or exhaust heating provided by heating elements to burn off the particulate matter stored within the DPF.
However, the inventors herein have recognized that during some conditions regeneration may cause the gases exiting the exhaust system and/or various components of the exhaust system to attain a substantially higher temperature. For example, temperatures exiting the exhaust system may be as high as 550° C., even during low engine output conditions, such as during idle. Further, some exhaust system components including a DPF and/or other aftertreatment devices may have a relatively high thermal inertia, thereby causing the exhaust gases and/or exhaust system to maintain an elevated temperature even after a regeneration operation has been completed.
One approach that attempts to reduce exhaust gas temperature is described in U.S. Pat. No. 6,973,959, where a heat exchanger device arranged in the exhaust passage may be used to extract heat from the exhaust gases flowing therein. In another approach, as set forth in U.S. Publication No. 2005/0205355, a converging nozzle/venturi device is used to cool the exhaust gases by adding ambient air into the exhaust system prior to being exhausted.
However, the inventors herein have also recognized that in the above executions, both of these approaches can generate more back pressure to the exhaust system upstream of the device than desired. The increased backpressure may result in reduced engine performance and/or efficiency.
In one approach, the above issues may be addressed by an exhaust system for an engine, comprising a first exhaust passage providing a first flow area; a second exhaust passage communicatively coupled to the first exhaust passage, the second exhaust passage providing a second flow area greater than the first flow area, wherein the second exhaust passage is arranged downstream of the first exhaust passage; wherein a first wall surface of the first exhaust passage defines at least a first opening for transferring air external the first exhaust passage to within the first exhaust passage and a second wall surface of the second exhaust passage defines at least a second opening for transferring air external the second exhaust passage to within the second exhaust passage; a first protrusion disposed within the first exhaust passage upstream of the first opening; and a second protrusion disposed within the second exhaust passage upstream of the second opening.
In this way, it may be possible to reduce the temperature of the gases exiting the exhaust system and/or reduce the temperature of various exhaust system components, such as those arranged downstream of the openings. The radial configuration of the air entrainment devices can result in a smaller increase in backpressure or backpressure penalty than may exist with similar devices arranged in series. The use of the radial arrangement can reduce the backpressure penalty for a given amount of entrained air due to the combined decrease in flow area achieved by the parallel grouping of entrainment devices. Further, by using entrained air both upstream and downstream of an expansion of the flow, the inventors herein have found that sufficient cooling of exhaust gases may be provided with a reduced backpressure penalty due to the synergistic effects of the pressure gain associated with the expansion and the improved efficiency of the entrainment device configuration.
While this approach may provide improved exhaust cooling with reduced backpressure, additional cooling approaches may be used, if desired. For example, heat exchangers and converging/diverging nozzles may still be used, if desired.